Half of All Farmed Fish Have Hearing Loss Due to Deformed Ear Bones

New research suggests the way we raise fish in farms and hatcheries could be causing harmful changes to their bodies.

A study published April 28 in the journal Scientific Reports found that half of the farm-raised fish examined had significant deformities in their ear bones, which are vital to hearing. And the chances for this deformity increase with age: All of the large 100-plus farm-raised salmon (weighing more than 9 pounds) had the deformity in at least one ear, says study first author Tormey Reimer, who completed the research as a graduate student at the University of Melbourne in Australia.

Fish with malformed ear bones, also known as otoliths, may lose as much as 50 percent of their hearing sensitivity, Reimer says.

“I’m rather shocked” by the ubiquity of the deformity, says Arthur Popper, a researcher at the University of Maryland who wasn’t involved in the study. It’s a “really important finding,” he adds.

In the paper, researchers examined more than 100 farm-raised and wild Atlantic salmon from Norway, Australia, Scotland, Canada and Chile; they also analyzed data from eight previous studies that looked at the otoliths of hundreds of coho salmon, lake trout and Atlantic herring.

The otoliths of these fish vibrate at a different frequency than the rest of the animal’s body and are used to pick up and measure sound waves, as well as to convert them into an electrical current that the brain processes. In healthy fish, these bones are composed of aragonite, a crystalline form of calcium carbonate. But as Reimer and colleagues found, many of these otoliths were deformed and composed of a different material—vaterite. Vaterite is also made up of calcium carbonate, but in an irregular and less dense crystal structure, thus changing the way that sound is absorbed and processed, Reimer says.

The presence of these vaterite deformities was 10 times higher in farmed fish than in wild animals, according to the study.

The ear bone on the right is deformed and made of a material called vaterite, while the one of the left is normal and made of aragonite. TORMEY REIMER

Exactly what’s causing the deformities is still a mystery. Farmed fish don’t appear to be born with these deformities; rather, something seems to cause them. Otoliths grow outward one layer at a time, like rings on a tree, and at some point in the case of the deformed fish, the animal’s body began laying down layers of vaterite instead of aragonite. Reimer says once this process is altered, the change is permanent.

“It only needs to happen once, and to our knowledge it’s irreversible,” she says. The researchers speculate it could be triggered by something in the fish’s diet, or may be a result of their abnormally rapid growth rate; farmed fish are often fed high-calorie foods to make them grow more quickly, Reimer says. Or there could have a genetic component, she adds; perhaps there’s something unique about the type of animals we’ve selected for in fish farms and hatcheries. The group hopes to test all these hypotheses in the near future.

Proper hearing is important for finding food, avoiding predators and navigation, so it’s likely that fish with deformed ear bones would have a reduced chance of survival in the wild, Reimer says. While most of these animals won’t see the wild, some fish (like Pacific salmon) may be raised in hatcheries before being released to the ocean, after which they hopefully return to be caught and eaten.

Allison Coffin, a researcher at Washington State University who wasn't involved in the study, says the deformity could possibly affect the survival of fish released to the wild but notes that salmon aren't known for having good hearing to begin with. "They aren't exactly champions of hearing," she says.

Importantly, though, the study provides "more evidence hatchery conditions are causing problems with the fish, and we need to figure out what we're doing," she says.

Popper’s group first discovered these vaterite abnormalities in hatchery-reared chinook salmon in the mid-2000s and published those findings in a 2007 paper. The study found that fish with deformed otoliths had significantly worse hearing than those without.